Process of etching metal with recovery or regeneration and recycling

ABSTRACT

THIS INVENTION REFERS TO A PROCESS WHEREBY THE REACTION PRODUCTS OF ETCHING COPPER, COBALT, IRON, NICKEL, ZINC, OR MAGNESIUM WITH CUPRIC CHLORIDE OR FERRIC CHLORIDE ARE PRECIPITATED AS A DOUBLE SALT OF THE DISSOLVED METAL CHLORIDE AND AMMONIUM CHLORIDE BY COOLING THE SOLUTION. THE DOUBLE SALT IS SEPARATED FROM THE REMAINING SOLUTION, THE   ETCHING SOLUTION IS RECOVERED AND REGENERATED BY THE USE OF AN OXIDANT SUCH AS CHLORINE, MURIATIC ACID AND HYDROGEN PEROXIDE, MURIATIC ACID AND AIR, OR MURIATIC ACID AND OXYGEN.

Aug. 17, 1971 WEGENER 3,600,244

PROCESS OF ETCHING METAL WITH RECOVERY 0R REGENERATION AND RECYCLINGFiled Feb. 20, .1969 3 Sheets-Sheet l ETCHING REACTION: 2E8 cw Cu 2% me+cu 0E2 REGENERATION REACTIONI 2Fe CL2 CL2"2FeCL3 COPPER PANELS INETCHED CIRCUITRY ETCHER NH4CL REGENERATOR CRYSTALLIZER CHLORINE 6A5DOUBLE SALT CuCL2-2NH4cL-2H20 BASIC FERRIC CHLORlDE RECOVERY SYSTEMBASIC CUPRIC CHLORIDE RECOVERY SYSTEM ETCHING REACTIONI cu (N2 cu -zcuCL REGENERATING REACTIONI ZCuCL +CL2 -2cucl 2 COPPERS PANELS IN EETCHEDCIRCUHRY E m H E R om m NH4CL CHLORINE GAS REGENERATOR CRYSTALLIZER 4DOUBLE SALT CuCL2'2NH4CL-2H20 MIX TANK INVEN/O/P HERBERT WEGENER AGENTAug. 17, 1971 Filed Feb. 20, 1969 H. WEGENER g 2 I g i ('TVS/ZOTNOIiVHlNl-TONOO HHddOO I I I I (NV ION) NOILVLHNHONOO LHddOf) 3,600,244PROCESS OF E'I'CHING METAL WITH RECOVERY 0R REGENERATION AND RECYCLING 3Sheets-Sheet 2 (MOLAR) TYPTCAL OPERATING CYCLE OF FERRIC CHLORIDECRYSTALLTZATION PROCESS WITH RESPECT TO MUTUAL SOLUBILITIES OF CuCL2 ANDNH4 CL IN 3.0 MOLAR Fe CL? SOLUTIONS PROCESS OF ETCHING METAL WITHRECOVERY OR REGENERATION AND RECYCLING Herbert Wegener, Endicott, N.Y.,assignor to International Business Machines Corporation, Armonk, N.Y.Filed Feb. 20, 1969, Ser. No. 800,979

Int. Cl. C23g 1/36; C23f 17/00; Hk 3/06 US. Cl. 15619 9 Claims ABSTRACTOF THE DISCLOSURE This invention refers to a process whereby thereaction products of etching copper, cobalt, iron, nickel, zinc, ormagnesium with cupric chloride or ferric chloride are precipitated as adouble salt of the dissolved metal chloride and ammonium chloride bycooling the solution. The double salt is separated from the remainingsolution, the etching solution is recovered and regenerated by the useof an oxidant such as chlorine, muriatic acid and hydrogen peroxide,muriatic acid and air, or muriatic acid and oxygen.

BACKGROUND OF THE INVENTION The invention is directed to an etchingsystem that permits the recovery or regeneration of a spent etchant andthe recycling of the regenerated or reconstituted etching solution, thusenabling the etching process to be carried out under steady stateconditions if so desired. More particularly, this invention relates to asystem wherein copper-cladded boards may be etched at a constant rate bycupric chloride or ferric chloride etchant and in which the spentetchant may be recovered by the removal of the copper salts therefrom,which salts are the product of the etching process, said recovered spentetchant being either refortified or regenerated for subsequent use.

In the printed circuit art, copper-cladded boards are etched accordingto a desired pattern to obtain electrically conductive circuitry. Thedesired conductive circuitry is obtained by coating the surface of theboards with an etch resist in the desired pattern leaving the unwantedmetal exposed. The removal of the unwanted metal, normally copper, isaccomplished by etching with a solution in which the metal is soluble.The removal of the metal from the board is generally performed in anetching chamber through which a succession of metal-cladded boards areconveyed and sprayed with an etchant to efiect the dissolution of themetal.

The etchant in the case in which copper is the metal to be dissolved isgenerally selected from ferric chloride, cupric chloride, or ammoniumpersulfate solutions. The etching proceeds according to the chemicalreaction Equations 1 for ammonium persulfate, 2 for cupric chloride, and3 and 4 for ferric chloride:

CuCl +Cu-9 2CuCl (Equation 2) 2FeCl Cu+ 2FeCl CuCl (Equation 3) CuCl+Cu+2CuC1 (Equation 4) With ferric chloride at lower copperconcentrations, e.g. 6 oz. 1 gal., dissolved copper exists in the cupricstate because it is oxidized by the ferric ion,

Fe+ +Cu+ +Fe+ -{-Cu" (Equation 5) and therefore, Equation 3 describesthe overall etching reaction at low copper concentrations.

The ammonium persulfate, cupric chloride, and ferric chloride etchingsystems are generally batch systems, i.e., systems in which the etchchamber is charged with a discrete volume of fresh etchant solution. Thefresh etchant United States Patent O ice is recycled from the bottom ofthe etching chamber and is sprayed or splashed onto the surfaces of thecoppercladded boards. The exposed copper is etched according to theabove chemical equation, and the reaction products are dissolved in theetchant. The etch rate is continuously reduced as the etchantconcentration diminishes and reaction products increase. The reaction iscontinued to a point where the rate becomes prohibitively slow. At thispoint, the etchant, containing both reaction products and unusedetchant, is dumped into waste treatment facilities and the system isrecharged with a fresh batch of etchant.

In batch etching systems the disposal of the unused etchant with thespent solution as Waste makes the process an expensive one. Further,steady state etching is, at most, difficult to maintain because as theetchant is contamined with the etching by products its concentration isreduced causing a decrease in the etching rate. To compensate forchanges in etching rates, it is necessary to continually vary the speedof the conveyor carrying the copper-cladded boards to insure removal ofthe exposed copper prior to the boards emergence from the chamber. Thisis necessary to avoid overetching and undercutting lines due tooverexposure and to provide efiicient machine utilization.

In those installations having batch etching systems from which the spentetchant is disposed as waste into streams and rivers, there arises theadditional problem of stream pollution. In order to overcome thisproblem, it is imperative that the copper salts are removed from thespent etchant prior to its discharge into the streams or rivers.

SUMMARY OF THE INVENTION In the past ammonium persulfate etching systemshave been used wherein the double salt of the reaction products wereremoved and the remaining etching solution recovered and fortified withammonium persulfate. Although ammonium persulfate systems were rathercostly and did not produce as good a quality etch as was desired, theywere used due to the fact that the copper-cladded boards were providedwith a tin-lead plating resist which was not affected by the ammoniumpersulfate. The tin-lead resist was ultimately replaced by an organicresist and it was then found that superior steady state etching ofprinted circuit boards could be carried out. In accordance with thepresent invention, this is accomplished through the combination of anetching system and an etchant recovery system wherein the etchant usedmay be either ferric chloride or cupric chloride. The ultimate conceptin etching is a continuous operation in which a metal is removed fromthe etchant at a rate in which it is dissolved into the etchant and theetchant consumed during the etching process is replaced at an equivalentrate. These operations have been performed with the present ferricchloride and cupric chloride systems, said systems being characterizedby having constant etching characteristics. The present systems provideadvantages over prior art systems in that chemical costs are very lowbecause the only chemicals regularly added to the system are ammoniumchloride salt and chlorine gas, higher quality etching is obtained withless undercutting whereby better finished boards are produced, and thesystems are better adapted for either continuous or batch operation.

Therefore, an object of this invention is the provision of an improvedetching system which overcomes the problems of the existent systems.

Another object of this invention is the provision of a continuousetching system in which steady state etching may be accomplished withoutthe need for continuously adjusting the conveyor speed.

Yet another object of this invention is the provision of a steady stateetching system in which the reaction prodnets are removed and the spentetchant is refortified or regenerated and reused.

Still another object of this invention is the provision of a steadystate etching system in which about 90% or more of the etchant may beadvantageously utilized.

Yet another object of this invention is the provision of a steady stateetching system in which the etch rate will remain constant.

In addition to the outstanding advantages of obtaining steady stateetching, this invention provides additional advantages such as the moreefficient utilization of the etchant, e.g., about 90% or more of theetchant is utilized in the system of this invention. Additionally, theetching reaction by-products are removed as solids in the form ofsaleable salts, thus eliminating the handling of large quantities ofsolution. Further, the problem of stream pollution is no longerexistent, since the contaminating agents are removed as the above statesaleable salts.

The foregoing and other objects, features, and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic flow diagram ofa preferred basic ferric chloride recovery system.

FIG. 2 is a diagram illustrating a typical operating cycle of the ferricchloride crystallization process shown in FIG. 1.

FIG. 3 is a schematic fiow diagram of a preferred basic cupric chloriderecovery system.

FIG. 4 is a diagram illustrating a typical operating cycle of the cupricchloride crystallization process shown in FIG. 3.

DESCRIPTION OF PREFERRED EMBODIMENTS The system concept can besummarized by the schematic presentations shown in FIGS. 1, 2, 3, and 4for the ferric chloride and cupric chloride recovery systems.

In FIG. 1, ferric chloride having an initial predetermined molarconcentration is flowed across a metal part, e.g. copper, in an etcherand an increase in copper concentration and a decrease in ferricchloride concentration thereby results. The solution is then flowedthrough a crystallizer where a portion of the copper is removed from thesolution in the form of a double salt,

CuCl 2NH Cl- 2H O It is desirable to add ammonium chloride, in an amountequivalent to that which will precipitate out in the double salt, in thecrystallizer as shown. The solution is next sent to a regenerator wherean oxidant such as chlorine gas is used to regenerate the etchant. Atypical operating cycle is presented on a stepwise basis in FIG. 2.Mutual solubilities of cupric chloride and ammonium chloride in 3.0molar ferric chloride are presented for a temperature of 70 F. Aconventional 34 Baum ferric chloride etchant contains approximately 3.0molar ferric chloride and 0.55 molar ammonium chloride and correspondsto point A on FIG. 2.

Copper is dissolved during etching to a concentration of approximately 6oz./gal., shown as point B. At this copper concentration, the etch rateis nearly the same as that of fresh etchant A. After etching, ammoniumchloride is added and the etchant corresponds to point C. Duringcrystallization, the solution is cooled to 70 F etched copper is removedas CuCl -2NH Cl-2H O, and the solution returns to point D. After aregeneration step, copper is again dissolved to point B and the cycleBCD repeated with regeneration on each pass. With the ferric chloridesystem, heat generated during chlorination raises the etchant to therequired etching temperature.

Two important points should be noted concerning the addition of ammoniumchloride. First, the quantity of ammonium chloride added must beequivalent to that re- 4 moved in crystallizing the double salt and thisamount can be predicted from data in FIG. 2. Second, ammonium chloridecan be added either before or during the crystallization step. In mostcases, it is desirable to add ammonium chloride during crystallization.

A ferric chloride crystallization system can be operated on a batch,semi-continuous or continuous basis and can tolerate relatively highamounts of etched copper without significantly decreasing the etch rateor forming salt crystals in the etcher. The ferric crystallizationsystem requires control of both copper and iron concentrations. Perhapsthe ferric system is best adapted to a batch process be cause of theconveniently high copper concentration that can be tolerated and therequirement that both copper and iron concentrations be controlled.

In FIG. 3, cupric chloride having an initial predetermined molareoncentration is flowed across a metal part, e. g. copper, in an etcherand an increase in cuprous chloride concentration and a decrease incupric chloride concentration thereby results. A portion of the solutionis then flowed through a crystallizer where a portion of the copper isremoved from the solution in the form of a double salt, CuCl -2NH Cl'2HO. It is desirable to add ammonium chloride, in an amount equivalent tothat which will precipitate out in the double salt, in the crystallizer.The solution leaving the crystallizer is mixed with that portion of thesolution leaving the etcher that did not pass through the crystallizerin the mix tank as shown in FIG. 3. The echant is regenerated with anoxidant such as chlorine gas either before or after crystallization.

The cupric etching system cannot tolerate relatively high amounts ofetched copper without significantly decreasing the etch rate. Tomaintain a high etch rate with the cupric chloride etchant, the amountof etched copper in the system should be kept at a low level. The lowcopper level requires a high solution flow rate to and from the etcher.This requirement increases the heating and cooling requirements betweenthe 120 F. etcher and the 70 F. crystallizer. The heat transferrequirements for the cupric chloride system can be substantially reducedby using the process flow shown in FIG. 3. Here, the crystallizer isplaced in parallel with a larger, uncooled, uncrystallized stream. Aftercrystallization, the streams are mixed, regenerated, and recycled to theetcher.

Sufficient ammonium chloride is added to the crystallizer to reduce thecopper concentration by approximately 4 oz./ gal. Because no copper isremoved from the bypass stream, the ratio of crystallized touncrystallized streams can be controlled to provide the desiredconcentrations in the mix tank. The following condition must besatisfied:

FeACu,,=F,,ACu where F=solution flow rate Subscript e=condition acrossthe etcher Subscript c=condition across the crystallizer This techniquepermits high solution flows and a low copper concentration increaseacross the etcher and simultaneously low solution flows and a highcopper concentration increase across the crystallizer. Heat transferrequirements are substantially reduced with no detrimental effect on theetching operation.

FIG. 4 shows the step-wise operating cycle for the cupric chloridesystem using the by-pass technique. Mutual solubilities of cupricchloride and ammonium chloride in 0.1 molar hydrochloric acid are shownat 70 F. Point A represents an initial or reprocessed etchant. Copper isetched to point B. A small portion of solution B is passed through thecrystallizer, ammonium chloride is added, and the solution crystallizedat 70 F. to point D. Addition of ammonium chloride can be madesimultaneously with crystallization. Subsequent mixing of crystallizedsolution D with the remainder of spent etchant B returns the mixture topoint A. After regeneration, etchant A is returned to the etcher.

Although FIG. 4 shows a crystallization temperature of 70 F., it may bedesirable to crystallize at a temperature of 50 F. or 60 F. A lowercrystallization temperature would allow reprocessed etchant A to remainunsaturated at room temperature and the etchant can then be stored underambient conditions without crystallization.

For carrying out the present invention as it relates to ferric chloridesolutions, a fresh aqueous 3 molar (M) ferric chloride solution is madecontaining about 487 grams of ferric chloride per liter of solution and294 grams of ammonium chloride per liter of solution.

The etching solution is then heated to a temperature of about 55 C. andused to etch printed circuit panels.

Etching is continued until the ferric chloride concentration decreasesto about 1.6 molar in ferric chloride. At this point, the solution iscapable of further etching but the etch rate and quality of etchdiminishes.

The spent solution is treated in accordance with the present inventionby adding about 1 mole of ammonium chloride and cooling the solution toabout 21 C. whereupon a copper chloride and ammonium chloride doublesalt crystallizes and precipitates from the solution. The exacttemperatures and concentrations are not critical, but it is desired thatit be such that cupric chloride and ammonium chloride double saltprecipitate out without substantial amounts of ferric or ferrouschloride.

The precipitated salts can be collected in a chamber and the solutiondrained from the crystals.

The resulting solution, about 1.6 molar in ferric chloride, and 1.4molar in ferrous chloride, 0.24 molar in cupric chloride, and 0.55 molarin ammonium chloride, is sent to the chlorinator where the ferrouschloride is oxidized to ferric chloride. The resulting solution, about3.0 molar in ferric chloride, 0.24 molar in cupric chloride, and 0.55molar in ammonium chloride, is returned to the etcher. When the ferricchloride concentration is decreased to 1.6 molar, the solution is sentto the crystallizer where ammonium chloride is added and the solution iscooled as indicated above.

It is possible to add the ammonium chloride either be fore or during thecrystallization step. It is desirable, however, to add the ammoniumchloride during the crystallization step since the addition of ammoniumchloride results in the precipitation of more cupric chloride andammonium chloride than for the case where the addition of the ammoniumchloride follows the crystallization ste F or carrying out the presentinvention as it relates to cupric chloride solutions, a fresh aqueous2.54 molar cupric chloride and 1.75 molar ammonium chloride solution ismade containing about 342 grams of cupric chloride per liter ofsolution, and 94 grams of ammonium chloride per liter of solution. Theetching solution is then heated to a temperature of about 40 C.-55 C.and use to etch printed circuit panels.

Etching is continued until the copper concentration reaches about 2.62molar. At this point, the solution is capable of further etching butthis etch rate and quality of etch diminishes.

The spent solution is treated in accordance with the present inventionby adding about 0.8 mole of ammonium chloride per liter of solution andcooling the solution to about 21 C. whereupon copper chloride andammonium chloride crystallize and precipitate from the solution. Theexact temperatures and concentrations are not critical but it is desiredthat it be such that cupric chloride and ammonium chloride precipitateout without substantial amounts of cuprous chloride.

The precipitated salts can be collected in a chamber and the solutiondrained from the crystals.

The resulting solution, about 1.75 molar in ammonium chloride and 2.54molar in copper, is sent to the chlorinator where the cuprous chlorideis converted to cupric chloride. The resulting solution, about 2.54molar in cupric chloride and 1.75 molar in ammonium chloride, is

6 sent to the etcher. When the copper concentration is increased toabout 2.62 molar, the solution is sent to the crystallizer whereammonium chloride is added and the solution is cooled as indicatedabove.

Although the above cooling temperature was indicated as 21 C., it may bedesirable to crystallize in a temperature range of 10 C. to 15 C. Alower crystallization temperature would allow reprocessed etchant toremain unsaturated at room temperature. The etchant can then be storedunder ambient conditions without crystallization.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

What is claimed is:

1. A process for precipitating the reaction products of etching copperwith ferric chloride in an etching solution containing ferric chloride,ferrous chloride, cupric chloride and ammonium chloride which comprisesthe steps of cooling the solution to 21 C. to crystallize the doublesalt of cupric chloride and ammonium chloride from the solution withoutcrystallizing large amounts of ferric chloride or ferrous chloride;

separating said double salt from the remaining solution;

adding ammonium chloride to the solution in an amount equivalent to thatwhich precipitated out in the double salt; and

regenerating the solution by the use of an oxidant such as chlorine,muriatic acid and hydrogen peroxide, muriatic acid and air or muriaticacid and oxygen.

2. A process according to claim 1 wherein before or during cooling ofthe solution, said ammonium chloride is added in an amount equivalent tothat which will precipitate out in the double salt.

3. A process for continuous etching with an aqueous 3 molar ferricchloride solution which comprises the steps of:

etching copper with an etching solution containing ferric chloride,ammonium chloride, ferrous chloride, and cupric chloride;

adding ammonium chloride to said solution;

cooling the solution to 21 C. to crystallize the double salt of cupricchloride and ammonium chloride without crystallizing large amounts offerric chloride or ferrous chloride;

said ammonium chloride which was added to the solution being added in anamount equivalent to that which precipitates out in said double salt;separating said double salt from the solution; regenerating the solutionby the use of an oxidant such as chlorine, muriatic acid and hydrogenperoxide, muriatic acid and air, or muriatic acid and oxygen; and

recycling the regenerated etching solution back to the etcher.

4. A process according to claim 3 wherein after cooling said solution,said ammonium chloride is added in an amount equivalent to that whichprecipitated out in the double salt.

5. A process for precipitating the reaction products of etching copperwith cupric chloride in an etching solution containing cupric chloride,cuprous chloride and ammonium chloride which comprises the steps of;

cooling the solution from 10 C. to 21 C. to crystallize the double saltof cupric chloride and ammoium chloride from the solution Withoutcrystallizing large amounts of cuprous chloride;

separating said double salt from the remaining solution;

adding ammonium chloride to the solution in an amount equivalent to thatwhich precipitated out in the double salt; and

regenerating the solution by the use of an oxidant such as chlorine,muriatic acid and hydrogen peroxide, muriatic acid and air, or muriaticacid and oxygen.

6. A process according to claim 5 wherein before or during cooling ofthe solution, said ammonium chloride is added in an amount equivalent tothat which will precipitate out in the double salt.

7. A process for continuous etching with an aqueous 2.54 molar cupricchloride and 1.75 molar ammonium chloride solution which comprises thesteps of;

etching copper with an etching solution containing cupric chloride,cuprous chloride and ammonium chloride;

adding ammonium chloride to said solution;

cooling the solution to 21 C. to crystallize the double salt of cupricchloride and ammonium chloride from the solution without crystallizinglarge amounts of cuprous chloride;

said ammonium chloride which was added to the solution being added in anamount equivalent to that which precipitates out in said double salt;

separating said double salt from the solution;

regenerating the solution by the use of an oxidant such as chlorine,muriatic acid and hydrogen peroxide, muriatic acid and air, or muriaticacid and oxygen; and

recycling the regenerated solution back to the etcher.

8. A process according to claim 7 wherein said solution is cooled from10 C. to 21 C.

9. A process according to claim 7 wherein after cooling said solution,said ammonium chloride is added in an amount equivalent to that whichwill precipitate out in the double salt.

References Cited UNITED STATES PATENTS JACOB H. STEINBERG, PrimaryExaminer US. Cl. X.R. 156-3; 25279.l

